Hoping maybe Andrew can comment....in a sub that uses opposed drivers, is it measured the same way as a conventionally designed sub or does it require some other method since the transducers are firing in opposite directions?

JohnK is correct. The only change that we have to make from a measurement standpoint is to measure with a microphone at a point equidistant from both woofers. For a subwoofer in a 4-pi environment, like on our outdoor measurement tower, we don't need to be too concerned as the frequencies we are dealing with are omnidirectional anyway.